'arclength' originates from Modern English, specifically the words 'arc' and 'length', where 'arc' ultimately comes from Latin 'arcus' meaning 'bow, arc' and 'length' comes from Old English 'lengðu' meaning 'length, extent'.

'arc' changed from Latin 'arcus' to Old French 'arc' and then into Middle English 'arc'; 'length' comes from Old English 'lengðu' (from Proto-Germanic *lengaz). The technical compound 'arc length' arose in English mathematical usage and has sometimes been written as the single word 'arclength' in modern texts.

Initially, 'arc' primarily meant a bow (curved shape) and 'length' meant extent; over time 'arc' came to denote a curved segment and the compound came to mean specifically the distance measured along that curved segment ('arclength').